1. Field of the Invention
The invention relates to a fuel cell system.
2. Description of the Related Art
Solid polymer type fuel cells have a fuel cell stack which is formed by laminating a plurality of cells (membrane electrode assemblies) and fastening them between end plates with through bolts. The cells are each composed of a solid polymer electrolyte membrane, an anode integrally joined to one side of the membrane, and a cathode integrally joined to the other side. The cells, when laminated, are each interposed between a plate that has a fuel channel of concave groove shape in its side facing toward the anode and a plate that has an oxidant channel of concave groove shape in its side facing toward the cathode. Fuel (hydrogen or reformed gas chiefly containing hydrogen) is then passed through the fuel channel, and an oxidant (air, typically) through the oxidant channel, so that an electrochemical reaction occurs via the solid polymer electrolyte membrane to generate electric power.
In such solid polymer type fuel cells, the solid polymer electrolyte membranes function properly when in a saturated humidity condition. Thus, the reactant gas(es) (fuel and/or oxidant) is/are humidified in a humidifier or the like before passed through the channels of the plates, whereby the solid polymer electrolyte membranes are maintained in the saturated humidity condition. The solid polymer type fuel cells have operating temperatures of approximately 80° C., whereas the electrochemical reaction, an exothermic reaction, can cause a rise in temperature during power generation. To avoid this, fuel cell stacks typically incorporate cooling plates, through the channels of which a coolant is circulated to maintain the fuel cell stacks within the operating temperatures.
In conventional fuel cell systems for operating a solid polymer type fuel cell, the coolant flow has been stopped in a system stopping process (see Japanese Patent Laid-Open Publication No. 2004-296340).
When the coolant flow is stopped in a system stopping process and the fuel cell stack is let to cool down naturally, the fuel cell stack starts to cool from outside. This causes temperature differences inside the fuel cell stack. Since water vapor in the fuel cell stack starts to condense at locations of lower temperatures, the individual cells vary in the water distribution in the process of natural cooling. As a result, the distribution of the reactant gases to the cells at the next startup becomes uneven, thereby making the cell voltages during power generation unstable. This may cause shortage of reactant gas and degradation of some cells. In some cases, cells are prevented from starting properly and the operation thereof is stopped when a protective function is activated. Besides, when the fuel humidifier or air humidifier pipe-connected has a temperature (water temperature) higher than the cell temperature, vapor can diffuse from the fuel humidifier or air humidifier into the cells and condense in the cells even if the reactant gas flows are stopped. This also changes the water distribution in the cells. Similarly, the distribution of the reactant gases to the cells at the next startup may become uneven, thereby degrading the cell and stopping the system.